Spellchecker

TortoiseSVN uses the Windows spell checker if it's available (Windows 8 or later).
Which means that if you want the spell checker to work in a different language
than the default OS language, you have to install the spell checker module
in the Windows settings (Settings > Time & Language > Region & Language).

TortoiseSVN will use that spell checker if properly configured with the tsvn:projectlanguage
project property.

In case the Windows spell checker is not available, TortoiseSVN can also
use spell checker dictionaries from
OpenOffice
and
Mozilla.

The installer automatically adds the US and UK English dictionaries.
If you want other languages, the easiest option is simply to
install one of TortoiseSVN's language packs. This will install
the appropriate dictionary files as well as the TortoiseSVN local
user interface.
After the installation finishes, the dictionary will be available too.

Or you can install the dictionaries yourself. If you have OpenOffice or
Mozilla installed, you can copy those dictionaries, which are located
in the installation folders for those applications.
Otherwise, you need to download the required dictionary files from
http://wiki.services.openoffice.org/wiki/Dictionaries.

Once you have got the dictionary files, you
probably need to rename them so that the filenames only have
the locale chars in it.
Example:

en_US.aff

en_US.dic

Then just copy them into the %APPDATA%\TortoiseSVN\dic folder.
If that folder isn't there, you have to create it first.
TortoiseSVN will also search the Languages sub-folder of
the TortoiseSVN installation folder (normally this will be C:\Program Files\TortoiseSVN\Languages);
this is the place where the language packs put their files.
However, the %APPDATA%-folder doesn't require administrator privileges and, thus, has higher priority.
The next time you start TortoiseSVN, the spell checker will be available.

If you install multiple dictionaries, TortoiseSVN uses these rules
to select which one to use.